Sorry this is happening to you. I am dealing with a very similar scenario - on the onset - and receive almost daily phone calls from my in laws who go as far as to leave threatening voice mails on my cell phone. Just got another one from the mother in law hours ago.
I wonder if there are more drastic measures that can be taken to prevent this from taking place in the future? With children involved there has to be something that can be done in order for this harrasement to stop immediately.